Recently, the needs for detecting a weak magnetic field such as detecting the bio-magnetic field, flaw detection, non-destructive detection or the like are increasing.
When detecting a minimal magnetic field, since the influence of environment magnetic field, such as earth magnetism entering into the detecting target or magnetic field produced from detection device, becomes trouble, various technical solutions to cancel the environment magnet field have been provided.
According to Patent Document 1 (JP2009-297224A), an environment magnetic field detection sensor and a detection sensor are placed in the several coils, which are set to reduce the influence of the environment magnetic field, so that the environment magnetic field is eliminated while the magnetic field of the detection object is measured.
According to Patent Document 2 (JP2012-152515A), the difference, between the output of a magnetic sensor containing the environment magnetic field and the magnetic field of the detection object and that of the magnetic sensor whose environment magnetic field is only detected, is obtained through calculation, and thus elimination of the environment magnetic field is realized and the detection magnetic field of the object is outputted. Here, differential operation of a pair of differential operation part, that is to detect the environment magnetic field and the magnetic field of the detection object respectively, and differential operation of the output of a pair of the differential operation part is conducted, and thus three circuits are needed in the differential operation circuit.
In addition, generally speaking, there is a problem of the so-called temperature drift, which is caused by fluctuation of the output of the magnetic sensor resulted from the change of the environment temperature of the magnetic sensor that detects the environment magnetic field. Especially, in the case of using a magnetoresistance element in the magnetic field detection element, output variation (temperature drift) of the magnetic sensor is easily caused by the change of the environment temperature. It is caused by resistance changes of the magnetoresistance element resulted from the change of the temperature.